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PARIS — France’s justice minister vowed Thursday to quickly capture a group that is extorting the government, threatening to blow up French railway tracks unless it receives millions of dollars in ransom.

“Everyone is mobilized,” Dominique Perben told RTL radio station. “I hope that we will very quickly identify the people behind it.”

The government, meanwhile, faced criticism for keeping the threats secret as it pursued an investigation of bomb threats that Cabinet members have described as serious.

“Once more, the law of silence has prevailed,” the Federation of Transport Users, a commuter’s advocacy group, said in a statement. “The French people should have been informed of the threat, which was known and being taken seriously.”

The threats, which were first made public Wednesday, appeared in letters sent to the offices of French President Jacques Chirac and Interior Minister Nicolas Sarkozy on Dec. 10, Feb. 13 and 17.

The government had earlier urged media not to report the extortion effort to protect efforts to establish contacts with the group. But the Interior Ministry released details Wednesday after the story leaked.

To ease commuter’s concerns, 10,000 rail workers searched thousands of miles of train tracks Wednesday to inspect for bombs or signs of foul play. Security on the network was heightened but service was not interrupted.

Information from the group, which identifies itself as AZF and was previously unknown to authorities, led to the recovery Feb. 21 of a sophisticated explosive device buried in tracks near Limoges in central France.

Police do not believe the group is connected to Islamic terrorism.

The group’s initials, AZF, are those of a chemical factory that exploded, killing 30 people, in southwestern France in 2001. Investigators believe that explosion was accidental.

The threat began with a letter on Dec. 11 from a self-styled “pressure group of terrorist character formed in a secular fraternity,” the Paris daily Le Monde said in a detailed report in its edition on Wednesday afternoon.

The group denounced the government, the school system and media, which it said were too close to power. It also threatened unspecified attacks, the paper said.

The group’s demands became more precise in its third letter on Feb. 13 threatening to bomb the railway if it were not paid the ransom.

The group said it would detonate a bomb on Feb. 18, but it did not, Le Monde said.

Communication through classified adPolice then placed a coded classified advertisement in a Paris daily and the group answered the next day with GPS coordinates to find the bomb buried near Limoges.

To pick up its ransom, the group first demanded police allow it to land a helicopter on top of the Montparnasse Tower, the tallest office building in Paris, but authorities said that would not be possible.

The group then came up with an alternate plan, instructing authorities to place the payment under a blue tarpaulin lying in a field south of Paris, but a police helicopter dispatched to the location could not find the site because of darkness, the newspaper reported.

It was not clear whether authorities had intended to pay the ransom or were setting a trap for the extortionists.

Sarkozy, the interior minister, blasted La Depeche du Midi for ignoring the appeal not to report the threats. “This newspaper’s behavior is not responsible,” he said. “Let the police do their job.”